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Figure 4. Lead-free reflow profile
Typical reflow profile used with lead-free SAC and SnAg alloys
The flux chemistry used in lead-free pastes is designed to minimise several of the issues associated with higher reflow temperatures. Higher temperature issues can range from increased paste slump to charring of the residues. Solder paste manufacturers are using chemicals that offer good hot slump resistance and activation at higher preheats and peak reflow temperatures.
Lower wetting speeds associated with alternative lead-free solders make flux activation a critical factor in paste performance. No-clean and water-washable solder pastes are being designed not to require nitrogen reflow, and can produce reliable solder connections with good wetting in air. Water-washable solder pastes, with their higher concentration of activators, will solder most metal finishes adequately. No-clean solder pastes will require careful selection of finishes to be soldered, as well as careful selection of a paste's attributes. Some lead-free, no-clean pastes are designed to adequately solder a variety of metal finishes. Others have difficulty with second-pass, bare copper boards, because of their lower activity. Some no-clean pastes require lower peak temperatures, while others can withstand higher temperatures without charring or polymerisation.
Nitrogen will affect solder joint cosmetics, as shown in Figure 5. Solder paste reflow in air will offer brighter, more uniform solder joint surfaces. Nitrogen reflow also will enhance wetting with lead-free solders - especially on bare copper OSP surfaces. It must be noted that the vast majority of assemblers seek a solder paste that can be reflowed in air, so many lead-free chemistries are being developed with this in mind.
 Figure 5. Impact of nitrogen on Sn-Ag-Cu solders
A test (shown in Figure 5) was achieved by printing lead-free solder paste on a white ceramic substrate, then reflowing one in air, and the other in nitrogen. Although the surfaces look dramatically different, this is only a surface reaction.
Selecting the best lead-free solder paste for the SMT process will be a critical variable, and the following can be used as a guide in the selection process:
* Print speed.
* Abandon time.
* Stencil life.
* Tack life.
* Solder ball test.
* Slump test.
* Spread test.
* Reflow window.
* Voiding potential.
* Double-reflow window.
* Cleanability.
* Pin testability.
It is unnecessary to repeat these tests in-house. Often, a good solder paste manufacturer will make the information available to the user. If the paste is chosen with care, and the SMT process is optimised, the lead-free transition will be achievable without jeopardising reliability and product yields. Common defects associated with lead-free include:
* Off-pad solder.
* Mid-chip solder balling.
* Tombstoning.
* Bridging (shorts) on fine-pitch QFP leads.
* Open joints.
* Non-wetting.
* Dewetting.
* Cold solder joints.
* Voids.
These defects can be avoided in a properly optimised process with a lead-free solder paste designed to provide adequate wetting, low slumping and low voiding with lead-free alloys.
Lead-free solders, because of their reduced wetting, will at times not completely wet the pad. This will be affected by flux activity and the use of nitrogen in reflow. Some assemblers, however, are modifying stencils to have less of an aperture reduction. This requires a solder paste with excellent hot slump properties to avoid bridging and solder balls. Other assemblers are seeking modifications to pad design. In many cases, this is not considered an issue.
Developing a lead-free SMT process requires good planning and a close working relationship with all suppliers. A team approach and proper training are essential for reliable lead-free integration - especially where dual systems (leaded and lead-free) are present.
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